Once you are a good programmer in any language you may be able to get productive in a different one of the same family (for instance, in Perl case, imperative object oriented languages) in a very short time.

Becoming a very good programmer in the other language is a completely different matter. It may take a long time because you will have to learn the idioms, quirks, libraries, culture, etc. For instance, becoming a very good C++ programmer even if you know Perl well would probably be a years task.

If the language is of a different kind, as for instance Haskell, then you know mostly nothing.